Re: [PATCH v4 1/2] tpm: CRB: Use ram_device for "tpm-crb-cmd" region

[Peter Maydell]

On Tue, 8 Feb 2022 at 15:56, Eric Auger <eric.auger@redhat.com> wrote:
>
> Hi Peter,
>
> On 2/8/22 4:17 PM, Peter Maydell wrote:
> > On Tue, 8 Feb 2022 at 15:08, Eric Auger <eric.auger@redhat.com> wrote:
> >> Representing the CRB cmd/response buffer as a standard
> >> RAM region causes some trouble when the device is used
> >> with VFIO. Indeed VFIO attempts to DMA_MAP this region
> >> as usual RAM but this latter does not have a valid page
> >> size alignment causing such an error report:
> >> "vfio_listener_region_add received unaligned region".
> >> To allow VFIO to detect that failing dma mapping
> >> this region is not an issue, let's use a ram_device
> >> memory region type instead.
> > This seems like VFIO's problem to me. There's nothing
> > that guarantees alignment for memory regions at all,
> > whether they're RAM, IO or anything else.
>
> VFIO dma maps all the guest RAM.

Well, it can if it likes, but "this is a RAM-backed MemoryRegion"
doesn't imply "this is really guest actual RAM RAM", so if it's
using that as its discriminator it should probably use something else.
What is it actually trying to do here ?

thanks
-- PMM